Fiber optic data transmission systems or links are used in certain telecommunications networks to transmit data over optical fibers between digital electronic devices. A fiber optic link offers many advantages over conventional electrical cable transmission of data, including faster transmission rates, increased information carrying capability, longer link distance, and lower cost. Typically these data transmission systems comprise a fiber optic transmitter such as a solid state light-emitting device, and an optical fiber for carrying the digital output of the fiber optic transmitter.
Solid state light-emitting devices commonly used in fiber optic data transmission systems include light-emitting diodes (LEDs) and laser diodes. An LED is a relatively low-power optical source which operates at a lower data transmission rate than does a laser diode due to a spectral output which is not suitable for high speed data transmission. For applications which require high-speed data transmission or which cover long distances, a laser diode is preferred as the optical transmitter. Typically, the full output of the laser diode is applied directly or through a focusing lens to the optical fiber.
The use of a laser diode in combination with an optical fiber, however, introduces a couple of problems. First, a portion of the laser diode output is reflected from the laser diode-optical fiber connection back toward the laser. This back reflection affects the operation of the laser by interfering with, and thus altering, the frequency and amplitude of the laser output oscillations. Second, the increased power associated with using a laser diode as an optical source introduces a concern for the safe operation of the data transmission system. These concerns include potential exposure of the laser output to the eye, particularly if the optical fiber is detachable from the transmitter, and operation of the system within prescribed power output specifications.
Methods are known to minimize laser diode back reflection caused by the laser diode output. A known manner of minimizing back reflection of laser output oscillations in a laser diode-optical fiber transmission system includes inserting an element intermediate the laser diode and the transmission fiber. Examples of such systems are shown in U.S. Pat. Nos. 4,372,644 to Unger and 4,807,954 to Oyamada, et al. Unger discloses a polarizing element intermediate the laser diode and the optical fiber to convert the linearly polarized wave at the laser output to a circularly polarized wave at the optical fiber input. In effect, the polarizing element acts as an isolator for suppressing back reflection of any light for which the polarization state is preserved upon reflection. Oyamada shows a lens element between the laser diode and the optical fiber to reduce the backward coupling efficiency of back reflected light, thereby substantially preventing reflection of the light beam from the fiber back to the laser. Neither Unger nor Oyamada, however, address power output considerations, as neither device shows means to significantly attenuate the output power of the laser diode. Moreover, neither Unger nor Oyamada suggest that an increased laser diode signal-to-noise ratio can be achieved by increasing the drive current supplied to the laser diode.
It is an object, then, of the present invention to provide a fiber optic data transmission system comprising a laser diode transmitter and an optical fiber, having an increased signal-to-noise ratio achieved by operating the laser diode at a higher drive current to increase its extinction ratio.
It is another object of the present invention to provide a laser diode transmitter for use in a fiber optic data transmission system, the optical power radiating from which is within prescribed safety limits.